In the unlikely event of a severe accident that damages the reactor core and breaches the containment, a nuclear power plant might release radioactive material into the environment. This release would include radiation particles in a continuous plume of steam and other gases. This plume of radioactive material would then disperse from the site through the environment and to the surrounding population, by expanding and moving downwind. Consequently, after modeling the onsite accident progression and mitigation measures (Step 2), the State-of-the-Art Reactor Consequence Analyses (SOARCA) project team used MACCS2 (a consequence assessment code) to model offsite release of radioactive material.

The MACCS2 code uses site-specific weather conditions, population data, and evacuation plans to calculate and model the radiation exposure of the population for the SOARCA project through the following pathways:

The SOARCA team did not, however, model the population's exposure as a result of eating food products on which radioactive aerosol particles may have settled. As part of the emergency planning in place around reactors, restrictions are placed on foodstuffs and agricultural products when there is a possibility of release. (See Step 4: Modeling Emergency Response for more information.)

Because MACCS2 is primarily a tool for probabilistic risk assessment (PRA), it accounts for the weather uncertainties that are inherent to accidents that might occur at any time in the future. Thus, the modeled results for an average person represent the average risk attributable to the variability in the weather.

The MACCS2 models showed that some of the calculated exposure would occur during the early phase of an accident, when the aerosol particles are being released from the plant buildings. Some of the exposure would also occur in the long-term, as a result of trace contaminants that remain after the land is decontaminated, or in lightly contaminated areas where people never had to evacuate or relocate. Depending on the relevant protective action guidelines and the level of radiation, such areas might be considered habitable.